This proposal is focused on elucidating the role of high density lipoprotein apolipoprotein A-l in reverse cholesterol transport as it relates to the prevention of inflammation and progression of atherosclerosis. We have produced and characterized a new double knockout animal model, the LDL receptor -/- and apoA-I -/- mouse in order to study and elucidate mechanisms by which HDL apoA-1 stimulates the removal of peripheral tissue cholesterol, modulates inflammation, and prevents the progression of atherosclerosis. Previous studies in hypercholesterolemic mice indicate that HDL apoA-I plays two important roles. The first, involves the removal of cholesterol from peripheral tissues preventing accumulation within the artery wall. Secondly, apoA-l serves to modulate the expression of adhesion molecules on endothelial cells lining the artery and modulates the induction of oxidative stress and inflammation. Preliminary studies from our group have shown that LDL receptor -/-, apoA-I -/- mice fed a Western diet containing 0.1% cholesterol and 10% fat show an approximate approximately 12-fold greater accumulation of cholesterol in peripheral tissues when compared to LDL receptor -/- only mice. Therefore, in this proposal we plan to test our main hypothesis that in the absence of plasma HDL apoA-I reverse cholesterol transport to the liver is severely limited, allowing for dramatic accumulation of foam cell derived cholesterol ester in peripheral tissues, the onset of inflammation, and the development of atherosclerosis. To test this hypothesis, we plan to measure cholesterol flux in vivo including; absorption, hepatic cholesterol synthesis and secretion, extrahepatic cholesterol synthesis, acquisition and cholesterol turnover in chow and diet-fed LDL receptor -/-, apoA-I -/- mice and compare to LDL receptor -/- and apoA-I -/- only mice. We also plan to test the hypothesis that inhibition of reverse cholesterol transport and the onset of inflammation in LDL receptor -/-, apoA-I-/- mice is reversible by restoring plasma HDL apoA-I concentration to LDL receptor -/-, apoA-l -/- mice using helper-dependent adenoviral technology. We believe that these studies will lead to important new information regarding the role of apoA-l in the initial stages of atherosclerosis development and define the link between hypercholesterolemia and inflammation and their respective roles in the pathogenesis of atherosclerosis.